Green’s Dictionary of Slang

a major lie.

1683		Dialogue between a Yorkshire Alderman and a Salamanca Doctor 2: At Langhorns Tryal, thou swor’st thou saw’st him Forty times in July following, that was a Rapper.
c.1698		B.E. Dict. Canting Crew n.p.: Rapper a swinging great Lie.
1725		New Canting Dict. [as cit. c.1698].
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. c.1698].
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
c.1850		Duncombe New and Improved Flash Dict. n.p.: Rapper a bouncing lie.
[1895		C. Crowe Aus. Sl. Dict. 65: Rapper, to speak vehemently].